Printed circuit boards may be used as components in electronic devices. PCBs may be made from multi-layer structures which typically comprise a conductive foil, such as copper foil, and a polymeric resin substrate. The conductive foil forms conductors while the polymeric resin substrate provides structural integrity and forms an insulation between the conductors. Since the conductor and insulator are in intimate contact, adhesion between the two contributes to the performance and reliability of the electronic devices made with them.
Electrodeposited and wrought or rolled copper foils used in the manufacture of printed circuit boards do not adhere well to the polymeric substrates. The prior practice for achieving adhesion between copper foil and insulating polymeric substrates has been to roughen the copper surface.
Surface roughening has been achieved by several means. The electrodeposited copper foils can be electroformed with a rough surface. On top of this rough surface further roughening is carried out by applying a high surface area treatment. These treatments may be a copper deposited electrolytically in nodular or powder form, or a copper oxide which grows nodular or dendritic, among others. Often times the rolled copper foil has mechanical roughness imparted to it during rolling or by subsequent abrasion. The rolled foils also are conventionally treated with surface area increasing nodular copper or copper oxide treatments.
These surface roughening treatments increase adhesion to the polymers by forming a mechanical interlock with the resin. The mechanical interlock is formed when an adhesive in its liquid state is applied and then cured or when the resin melts and flows prior to cure during lamination. The polymers flow around the roughened surface area treatments to form the mechanical interlock.
There are several factors contributing to the adhesion measured between the copper foil and the polymeric resin. Some of these are surface area, type of roughness, wettability, chemical bond formation, type of chemical bond, formation of interpenetrating networks, and properties of the adhering materials.
During an adhesion test the interlocked resin and copper often adhere well enough that failure occurs within the resin, a cohesive failure. With some resins the mechanical interlocking of treatment and resin does not result in the desired high adhesion and failure occurs at the interface between resin and copper, an adhesive failure.
Various different prepregs have been used to make the polymeric resin substrate. When an epoxy or polyimide prepreg is used, an adhesion promoting layer containing an epoxy silane has demonstrated satisfactory adhesive characteristics. For example, .gamma.-glycidylpropyltrimethoxysilane as a 0.5 percent by volume solution in water has been used with some success.
In general, the effectiveness of coatings in bonding dielectric substrates to metal foils depends upon the compatibility of the coatings with the foil and substrate. Since different foils and/or substrates may be used in a given multi-layer structure, it is necessary to determine compatible substrate-foil-adhesive combinations.
Polyimide resins have been used for the polymeric resin substrate because of their thermal properties. In fact, polyimide resins are used frequently because they possess high reliability for extended periods of time. More specifically, upon comparing polyimide resins and epoxy resins, polyimide resins exhibit higher glass transition temperatures and lower coefficients of thermal expansion. As a result, PCBs made with polyimide resins have reduced barrel cracking, improved pad repairability, and high reliability for high density circuit boards. One commonly used polyimide resin is derived from a bismaleimide of maleic anhydride and methylene dianiline (BMI/MDA). However, MDA is a suspected carcinogen and, accordingly, a number of problems are associated with its use. Since BMI/MDA polyimide resins are extensively used, adhesives compatible with BMI/MDA polyimide resins are likewise extensively used.
Recently, new prepreg resin systems have been introduced into the market place. Conventional adhesive systems heretofore used with BMI/MDA polyimide resins, epoxy resins, or any other conventional resins for that matter are not necessarily compatible with the new prepreg resin systems reaching the market place. Therefore, it is desirable to provide adhesives for use in multi-layer structures which can effectively bond conductive foils with the new prepreg resin systems described above. It is desirable that the adhesives display capability via strong adhesion to both the conductive foils and the new polyimide resins while simultaneously displaying high temperature stability.